1. Field of the Invention
The present invention generally relates to an optical display system and more particularly relates to a projection type optical display system including a liquid crystal panel.
2. Description of the Related Art
A projection type optical display system may conduct a full-color display operation either by a three-panel method (i.e., with three liquid crystal panels used for the three primary colors of red, green and blue, respectively) or by a single-panel method (i.e., with just one liquid crystal panel used).
A three-panel projection type optical display system includes: a color separation optical system for separating light, emitted from a white light source, into three light rays representing the three primary colors of red (R), green (G) and blue (B); three liquid crystal panels for modulating the R, G and B light rays in accordance with image information; a color synthesizing optical system for synthesizing together the R, G and B light rays that have gone out of the liquid crystal panels while carrying the image information; and a projection optical system for projecting the color-synthesized light rays with the image information. The three-panel projection type optical display system can efficiently utilize the light that is radiated from the white light source and ensures excellent color purity. However, the three-panel type needs a greater number of components than a single-panel type. Thus, the three-panel type is normally less advantageous than the single-panel type in respects of cost and size.
On the other hand, a single-panel projection type optical display system needs just one liquid crystal panel and a smaller number of components. Thus, the single-panel type can be manufactured at a lower cost than the three-panel type, and can be used effectively as a projection type optical display system of a reduced size. Japanese Laid-Open Publication No. 59-230383 discloses a single-panel projection type optical display system that conducts a display operation in full colors by providing color filters for a liquid crystal panel. In such a projection type optical display system, however, incoming white light is absorbed into the color filters and only one-third of the incoming white light can be used. Accordingly, compared to a three-panel type that uses a similar light source, the brightness of the image displayed decreases to about one-third in the single-panel type.
To overcome such a problem, Japanese Laid-Open Publication No. 4-60538 discloses a projection type optical display system that can display a brighter image with the optical efficiency increased without using any color filter. In this projection type optical display system, the white light that has been radiated from a light source is separated into R, G and B light rays by dichroic mirrors that are arranged in a fan shape. The light rays are then incident onto a microlens array at mutually different angles. These light rays that have been incident onto a microlens are transmitted through the microlens so as to be focused onto their associated pixels of the liquid crystal panel in accordance with their angles of incidence.
As used herein, the “liquid crystal panel” is an optical element that includes a liquid crystal layer with optical anisotropy and that modulates the light being transmitted therethrough when the optical properties of its liquid crystal molecules are changed upon the application of a voltage to the liquid crystal layer. The liquid crystal panel can display an image, a character, etc. thereon. When the liquid crystal panel is combined with a backlight, a direct-viewing type liquid crystal display is obtained. Also, when the liquid crystal panel is combined with a light source and a projection optical system, a projection type optical display system is obtained.
In the liquid crystal panel, multiple pixel electrodes are regularly arranged in matrix (i.e., in columns and rows), and drive voltages, representing image signals, are independently applied to the respective pixel electrodes. The drive voltages may be independently applied to the pixel electrodes either by a simple matrix addressing technique or by an active matrix addressing technique that uses nonlinear two-terminal or three-terminal elements. In the active matrix addressing technique, switching elements such as MIMs (metal-insulator-metal elements) or TFTs (thin-film transistors) and interconnect electrodes for applying drive voltages to the pixel electrodes are provided.
The liquid crystal panel typically operates in a twisted nematic (TN) mode or in an electrically controlled birefringence (ECB) mode, for example. Specifically, in a TN mode liquid crystal panel, liquid crystal molecules in a nematic liquid crystal layer, which is provided between a pair of transparent electrode substrates, are oriented so as to have a twist angle of approximately 90 degrees. In an ECB mode liquid crystal panel on the other hand, the liquid crystal molecules in the nematic liquid crystal layer have a twist angle of approximately 0 degrees. In the TN mode liquid crystal panel, the liquid crystal molecules may be twisted either dextrorotatorily or levorotatorily. Small-sized high-definition liquid crystal panels for use in head mounted displays (HMDs) or projection type optical display systems normally the TN mode (except for reflection types).
The liquid crystal panel is normally designed to display color black when the liquid crystal molecules in its liquid crystal layer are aligned vertically. However, when a linearly polarized light ray is obliquely incident onto the vertically aligned liquid crystal layer, the light ray is transmitted as an elliptically polarized light ray due to the birefringence of the liquid crystal molecules. And when this elliptically polarized light ray passes through a polarizer on the light outgoing side, linearly polarized components, having the same polarization direction as the polarization axis of the polarizer, are transmitted through the polarizer. As a result, leakage of light is created in the black state to decrease the contrast ratio.
As for a projection type optical display system in which incoming light is radiated substantially along a normal to the liquid crystal panel, such a decrease in contrast ratio is a negligible one. However, in the projection type optical display system disclosed in Japanese Laid-Open Publication No. 4-60538 identified above, the R, G and B light rays, which have been separated by the dichroic mirrors, are radiated onto the liquid crystal panel so as to define a tilt angle of about 8 degrees with respect to a normal to the liquid crystal panel. Thus, the decrease in the contrast ratio of the image is significant in such a situation. Also, the R, G and B light rays have mutually different angles of incidence. Furthermore, due to the variation in the refractive index of the liquid crystal material with the wavelength, the elliptically polarized versions of the R, G and B. light rays have mutually different spot shapes. Accordingly, the balance among the outgoing R, G and B light rays will be disturbed, thereby making it difficult to reproduce color white or creating color unevenness.
Furthermore, in the optical display system disclosed in Japanese Laid-Open Publication No. 4-60538 identified above, light with a sufficiently high degree of parallelism (of about ±4 degrees) must be used to avoid the mixture of the R, G and B light rays. Thus, the light that has been emitted from the light source cannot be used so efficiently as in the three-panel type (that uses light with a degree of parallelism of about 11 degrees). Furthermore, in the optical display system disclosed in Japanese Laid-Open Publication No. 4-60538 identified above, if a projection lens having an F value of about 1 is used, then a light receiving angle will be about 26 degrees and the light may have a degree of parallelism up to about ±9 degrees. However, when light enters the liquid crystal panel at as high a light receiving angle as 26 degrees, the leakage of light will be quite perceivable and the decrease in contrast ratio will be significant.
To overcome these problems, Japanese Laid-Open Publication No. 8-220521 discloses a projection type optical display system that increases the contrast ratio by providing a phase plate between a liquid crystal panel and a polarizer. On the other hand, Japanese Laid-Open Publication No. 9-258328 discloses a projection type optical display system that minimizes the wavelength dispersion by providing a phase difference compensator between an optical path shifter and a spatial light modulator (SLM) with a liquid crystal layer. In the projection type optical display systems disclosed in Japanese Laid-Open Publications Nos. 8-220521 and 9-258328, the incoming light is supposed to reach the light incoming plane of the liquid crystal panel or the spatial light modulator along a normal to that plane. Actually, though, the incoming light is not incident onto the light incoming plane exactly along a normal to that plane. In the optical display system disclosed in Japanese Laid-Open Publication No. 4-60538, in particular, the incoming light defines a predetermined angle with respect to a normal to the light incoming plane of the liquid crystal panel. Thus, the method disclosed in Japanese Laid-Open Publication No. 8-220521 or No. 9-258328 is not applicable to the optical display system disclosed in Japanese Laid-Open Publication No. 4-60538 to minimize the decrease in contrast ratio.
Japanese Laid-Open Publication No. 2001-174776 discloses a three-panel projection type optical display system, which includes three liquid crystal panels and an integrator optical system. In this projection type optical display system, light is incident onto one of the three liquid crystal panels so as to define a tilt angle of about 12 degrees with respect to a normal to the light incoming plane of the liquid crystal panel. The liquid crystal panel includes an optical compensator, which is made of a polymer layer where the optic axis defines variable tilt angles in the thickness direction. By providing this optical compensator, the quantity of leaking light, resulting from the obliquely incident light, can be minimized and the incidence angle dependence of the contrast ratio can be reduced.
However, a projection lens, which is normally used currently, has an F value of about 1.7 and a light receiving angle of about ±16 degrees. Also, in the liquid crystal layer described above, the greater the angle of incidence of the incoming light, the greater the quantity of light leaking therefrom. In view of these considerations, it is rather necessary to compensate for the phase difference of light with an angle of incidence of about 12 degrees to about 25 degrees.